Tubing used for oil well casings and drill pipe, for example, must have superior strength and hardness characteristics. This can be achieved with the use of high alloy steels, but such steels are expensive.
In order to harden and strengthen low alloy steels, processes have been developed in the past for quenching both the inner and outer surfaces of the tubing; but these prior art practices do not uniformly quench the inner surface of the tubing, resulting in non-uniform hardness along the cross section of the tubing wall, non-uniform hardness along the length of the tubing, and quench-cracking due to the non-uniform hardness characteristics.
Some prior art systems employ a free-flowing stream of quenching liquid flowing along the inner wall surface of a tube while it is in a vertical position, followed by quenching of the outer surface. Other prior art systems attempt to quench with the use of a circumferential outside diameter spray ring or longitudinal spray bars. This not only fails to achieve quenching of the inner surface but also requires water volumes of up to 5000 gallons per minute. Still another prior art method for quenching both the inner and outer wall surfaces of tubing is that shown, for example, in Heinenberg U.S. Pat. No. 2,888,374. In that patent, a quenching system is described wherein a cooling liquid is introduced into the tube in a vortical manner rather than as a free-flowing stream. While this system is perhaps better than the free-flowing quenchant system for interior wall quenching, the vortical water stream does not stay in contact with the inner wall of the tubing throughout its length. This results in non-symmetrical quenching of the inside diameter of the tubing since, in a typical 40-foot tube for example, drag between the quenchant liquid and the inner wall of the tube will reduce the speed of the quenchant which will, in turn, reduce the centrifugal force acting on it. As a consequence, the quenchant falls away from the inner diameter of the tube at the end opposite an injection nozzle which produces the vortical motion. The result is that the bottom half of the far end of the tube is quenched on its inner diameter while the top half of the inner diameter is not quenched.